A federal judge overstepped his authority by banning new plantings of genetically engineered alfalfa, according to a U.S. Supreme Court decision.

However, that doesn't mean farmers will immediately be allowed to grow the crop.

The decision overturns an injunction that blocked widespread cultivation of the crop, but the terms under which large-scale production can resume must be determined by USDA -- and probably cleared by a lower court.

"The bottom line is that nothing changes until the Department of Agriculture decides to take further action," said George Kimbrell, an attorney for the Center for Food Safety, which opposes the crop's commercialization.

However, the company that developed genetically engineered alfalfa, the Monsanto Co., is optimistic that USDA will seek partial deregulation and the process won't encounter lengthy delays.

"We will do everything we can to work with the government and other interested parties to accelerate that as much as possible," Monsanto attorney David Snively said during a conference call.

The USDA's Animal and Plant Health Inspection Service is capable of reconciling transgenic alfalfa production with the concerns of environmentalists and non-biotech farmers, said Mark McCaslin, president of Forage Genetics International, which helped commercialize the seed.

"We're pleased to have this back in APHIS's court, because they understand this coexistence," McCaslin said.

Beyond the specifics of alfalfa, the Supreme Court ruling will have repercussions for other biotech crops that encounter litigation, such as transgenic sugar beets, Snively said.

The ruling places limits on the power federal courts can exert over the regulation of such crops, he said. "You can't hijack the process through litigation."

Roundup Ready alfalfa, which is resistant to the herbicide glyphosate, was initially deregulated by the USDA in 2005, allowing farmers to grow and sell the crop freely.

Some farmers and environmental groups challenged the crop's deregulation in court, arguing that genes from Roundup Ready alfalfa would contaminate non-biotech crops and create herbicide-resistant weeds.

In 2007, U.S. District Judge Charles Breyer found that USDA's decision violated federal law and ordered the agency to conduct a thorough environmental study of the transgenic crop.

The judge precluded USDA from even partially deregulating the crop and banned most future plantings of the seed until the study was complete.

The nation's highest court reversed that injunction on June 21, ruling that Breyer abused his discretion by issuing an overly broad order when other measures could have prevented irreparable harm to the plaintiffs.

"An injunction is a drastic and extraordinary remedy, which should not be granted as a matter of course," wrote Justice Samuel Alito in the 7-1 majority opinion.

The Supreme Court only ruled on Breyer's injunction, not his decision to invalidate the USDA's deregulation of the crop, known as a "vacatur."

The agency must now decide to what extent Roundup Ready alfalfa should be cultivated before the environmental review process is done -- and that decision will still be open to legal challenge, the court said.

For the foreseeable future, the crop will remain off-limits to most farmers, Kimbrell said. "Whether you call that an injunction or a vacatur, I don't think we care either way."

The USDA is working to finish the environmental study ordered by Breyer, so it's unknown how much effort the agency is willing to expend on a partial deregulation plan in the meantime, McCaslin said.

"The ball is clearly in their court," he said.

In a draft version of the study, USDA recommended once again deregulating the crop completely. Forage Genetics hopes that will also be the conclusion of the final version.

"That is the final endgame to this," McCaslin said.

Breyer's injunction allowed farmers to grow Roundup Ready alfalfa if they had already bought or planted the seed. Under those terms, limited seed production took place in 2007, 2008 and 2009.

That inventory of unsold seed has created fears of a sudden oversupply if Roundup Ready alfalfa is deregulated, but McCaslin said he doesn't expect a major disruption.

Much of the alfalfa grown in the Midwest and Northeast is intermixed with grass, he said. Under this forage-type cultivation, Roundup Ready alfalfa wouldn't be viable, since the grass would be killed by the herbicide.

For that reason, Roundup Ready alfalfa probably won't be adopted as quickly as some other transgenic crops, thus preserving demand for conventional seed, McCaslin said. "It's not going to happen overnight."